We are sensitive to the temperature control of food. When it comes to food hygiene, temperature control first comes to mind of many people. General consumers are accustomed to putting groceries in the refrigerator immediately after bringing them home from a supermarket because they assume they would suffer from food poisoning by eating foods left out at room temperature.
As I mentioned in my column #3, cooling food is an earlier tradition of Japan and other countries. Its purpose is preserving foods for long periods of time.
What is important in food preservation is maintaining their quality, and inhibiting the proliferation of microorganisms is one of the key factors.
Now, above what temperature can microorganisms proliferate? Microorganisms shown in the figure below can proliferate below 10°C. Other than these, there are food-contaminating bacteria that can proliferate below 10°C including; Pseudomonas, Alcaligenes, Acinetobacter, Moraxella, Flavobacterium and Enterobacter of Gram-negative bacteria, and Micrococcus, coryneform group of bacteria, lactic acid bacteria and Actinomycete of Gram-positive bacteria.
That is to say, most foods cannot be preserved that long at 10°C. Then why refrigeration temperature is often designated as below 10°C? This temperature setting has the following background.
In 1952, a notice “On Hygiene Preservation of Orizume Bento (boxed lunch)” was published under the name of the section manager of Food Hygiene Section of Ministry of Health and Welfare. “Orizume” sometimes means “bento (packed lunch)” by itself, but I suppose the title is expressed as “orizume bento” because lunches of the day were often packed in wooden boxes called “oribako (folded box)” made of thicker sliced wood. In those days, rice balls were wrapped by thinner sliced wood, which might be even unimaginable for today’s young Japanese people.
Based on this notice, “On Hygiene Norms of Bento and Prepared Foods” was published in 1979 under the name of the above-mentioned section manager, and the preceded notice was abolished. The second notice adhered fundamentally to the previous one which was published just 7 years after the war. It was the era of refrigeration by ice, and refrigerators were not capable of cooling food to sufficiently low temperatures. I have heard from a person who was in charge of drawing up the notice that this is the reason why the reference temperature was set at 10°C.
The past standard also remains in the illuminance reference value designated in the second notice. The notice provides that the illuminance on all the working tables should be 100lx or more. This is because naked bulbs were predominant with little fluorescent lights in the days of the first notice, and it was virtually impossible to illuminate the working space brighter.
Today, facilities such as food manufacturing factories generally set an illunimance goal at 500lx or more, whereas those of international standards like Codex are 540lx.
Back to the refrigerating temperature in the second notice, 10°C was not the target figured out from microbiological viewpoint, but the lowest refrigerating temperature possible at that time.
The second notice actually made a supplementary statement that it was desirable to keep refrigerators below 5°C. However, the "Sanitary Management of Large Scale Cooking Facilities Manual" published in 1997 describes the setting temperature only as 10°C and the reason is unclear. In any event, international standards like Codex prescribes that food should be refrigerated below 4°C. If inhibition of microorganisms’ proliferation leads to food quality maintenance, it can be said that refrigerators should be kept below 4°C.